He only told that to the A rank party, everyone else is unaware of that. Now trying to steal it when he's being escorted by an A rank party, that's beyond stupid.
If they are so confident he's weak they could just wait until he takes a quest alone again.
Probably lowered the cost of having to buy armour along with the costs of maintenance. Also with the armour like that he can make his fighting style easily to incorporate guarding with that side while also twisting and turning around that shoulder leading into an counterattack he would like to do as a follow up.